In the automotive industry, there is a need for light-weight, high-strength structural members. Typically, automotive structures such as rails, pillars and the like are hollow to conserve mass while still providing high strength. Although structural members with these desirable characteristics can be readily obtained through the use of various exotic metal alloys, high-strength alloys are generally cost prohibitive in automotive applications.
Although filling the entirety of a section with plastic foam does significantly increase section stiffness (at least when high-density foams are utilized), this technique may also significantly increase mass and thus part weight, which, as stated, is an undesirable feature in most applications. In addition, filling a section entirely with foam can contribute significantly to cost and often creates an unwanted heat sink. Although increasing the metal gauge of a section or adding localized metal reinforcements will increase stiffness, as the metal thickness increases, it is more difficult to form the part due to limitations of metal forming machines.
A number of approaches have been proposed for dealing with the problem of reinforcing hollow sections subjected to buckling loads as alternatives to a high-cost alloys, thick gauge metals and large foam cores. For example, a reinforcement insert for a structural member comprising a precast reinforcement has been proposed wherein the reinforcement is formed of a plurality of pellets containing a thermoset resin and a blowing agent. The precast member is expanded and cured in place in the structural member. A composite tubular door beam reinforced with a syntactic foam core localized at the midspan of the tube has also been described in the art. The resin-based core occupies not more than one-third of the bore of the tube.
Tube-in-tube structures having high stiffness-to-mass ratios have also been proposed in which two nested tubes have a layer of foam disposed in the annulus between the tubes. A local reinforcement in the nature of a foamable resin disposed on a drop-in carrier has also been described. The carrier is placed in the channel of a hollow structural member and the resin is expanded.
Accordingly, low-cost techniques for reinforcing hollow sections without significantly increasing mass are quite desirable. Further, it is also desirable to provide methods of reinforcing existing hollow sections which do not require any fundamental design change to the member. The present invention provides hollow sections which have increased strength with moderate increases in mass, all without the use of large volumes of expensive resins. The present invention further provides a method for reinforcing existing structural parts without redesigning the geometry of the part or adding a laminate to the inside wall.